Device and method for the creation of droplet targets

ABSTRACT

An apparatus for making a droplet target provided with a chamber for receiving a target liquid and maintained under pressure, an electromagnetic valve switched at a ms rate for feeding target liquid from the receptacle to a heated expansion channel for converting the target liquid to supersaturated vapor and connected to a supersonic nozzle wherein the supersaturated vapor is cooled and condensed to droplets before they are discharged.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an apparatus for making a droplet targetprovided with at least one receptacle for receiving a target liquid andin which high pressure is generated by means of gaseous nitrogen, amagnetic valve connected to the receptacle and switchable in the msrange, and a nozzle, as well as to a method of forming a droplet target.

2. The Prior Art

Hereafter, devices known in the prior art will be described by whichliquid droplets are being generated wherein the interaction of laserbeams aimed at these droplets generates X-rays or extreme ultra-violetlight. Such rays are used, for instance, in microscopy and lithography.

U.S. Pat. No. 6,324,256 describing an arrangement of a laser plasmasource for generating EUV light, also refers to a device for makingdroplet targets. The droplets made are of a diameter larger than thediameter of droplets generated by a gas fed through a nozzle where itcondenses to form a cloud of clusters of extremely small particles. Asdescribed, a liquid is formed from the gas by means of a heat exchangerwhich reduces the temperature of the gas. The liquid is fed to a nozzlethe opening of which increases in the direction of the exit opening. Thedroplets are formed in this section and then exit from the exit openingof the nozzle to interact with a laser beam for generating EUV light.However, it is not possible in this arrangement in a defined manner toset the size of the droplets. In this arrangement the gaseous initialmaterial is converted to a liquid one. Moreover, the droplets interactwith the laser very close to the nozzle which in consequence of the heatand erosion is destroyed.

In Opt. Common. 103, 105 (1993), L. Ramble and H. M. Hertz report on anX-ray source in which droplets of ethanol are used as the target. Togenerate these droplets, ethanol was pressed at 30 to 50 at into avacuum chamber through a capillary of about 10 μm diameter tapering inthe direction of the nozzle. In order to generate a liquid volume—inthis case of a diameter of 15 μm—pressure surges were piezo-electricallyproduced at a frequency of about 1 MHz. The relatively large dropletswere used for examining the interaction with laser radiation in anintensity range of 10¹² to 10¹⁴ W/cm² as described by O. Hemberg, B. A.M. Henson, M. Berlund and H. M. Hertz in J. Appl. Phys. 88, 5421 (2000).Since in this case each individual droplet is interacting and the laserfocus is but slightly larger than the diameter of the droplets ofethanol, the drift problem of the droplet source is of major importance,the project is especially directed to solving an exact droplet-lasersynchronization.

Super dense droplet spray of a density of up to 10¹⁹ atoms/cm³ and adroplet diameter of about 1 μm was produced by a droplet sourcedescribed by L. C. Mountford, R. A. Smith and M. R. H. R. Hutchinson inRev. Sci. Instrum. 69, 3780 (1998) and is the basis of the instantinvention. The basis of this droplet source is a magnetic valve whichforms the pulse of liquid and, therefore, the volume of the liquid. Areceptacle was filled with a liquid and kept under high pressure bymeans of methanol. The valve is opened in synchronism with the laserpulse and for 2,500 μs to allow droplets to emerge from the nozzle. Itwas possible to produce droplets of lesser diameter of about 0.6 μm bysubsequent electrostatic cleaving of the droplets. This, however,requires a technically complex arrangement. However, the jog consistingof such droplets is of lower density, viz. about 10¹⁶ atoms/cm³.

For effectively generating X-rays or EUV light it is necessary, however,to make available droplet targets of dimensions of the size of possiblelaser wavelengths (T. D. Donelly, M. Rust, I. Weiner, M. Allen, R. A.Smith, C. A. Steinke, S. Wilks, J. Zweiback, T. E. Cowan, and T.Ditmire, J. Phys. B: At. Mol. Opt. Phys. 34, L313 (2001)) and,therefore, of a smaller diameter compared to the prior art, and whichform a spray of an atomic density of >10¹⁸ atoms/cm³.

OBJECT OF THE INVENTION

It is thus an object of the invention to provide a way by which suchdroplet targets can be produced. The high density is also to be realizedat a greater distance from the nozzle, i.e., the droplet target,compared to the prior art, is of a superior collimation in order toextend the useful life of the nozzle.

SUMMARY OF THE INVENTION

The object is accomplished with an apparatus of the type referred tosupra in which the nozzle, in accordance with the invention, isconstituted by an supersonic nozzle, the vale is connected to thesupersonic nozzle by an expansion channel, heating means are formedaround the expansion channel such that the temperature may be set at alevel at which a super saturated vapor is generated in the expansionchannel, and an insulation is provided between the electromagnetic valveand the heating means.

The apparatus in accordance with the invention makes possible thegeneration of super dense sub-μ liquid targets required for examiningthe interaction between laser radiation and plasmas. In contrast to thementioned prior art generating droplets in the saturated gas phase, thedroplets in accordance with the invention are generated from supersaturated vapor which condenses into a cloud of spray. The targetgenerated by the apparatus of the invention consists of droplets of amean diameter of about 150 nm and is of a mean atomic density of >10¹⁸atoms/cm³. Such a target makes possible the examination of conditions,not hitherto researched, which exist between clusters (from severalatoms to 10¹⁶ atoms/cluster to a local density approximating that of asolid) and solids. Moreover, relative to the advantages of a clustertarget, the spatial extent of the droplets influences an increasedvolume charge limitation of hot electrons which, in turn, results in animproved coupling of the laser energy with the ions of the droplets.Thus, a much hotter plasma can be generated and the effect in the X-rayconversion can be improved. The droplet target produced with the deviceof the invention can be generated continuously and, in terms of time, isof unlimited operation.

Embodiments of the apparatus in accordance with the invention relate tothe structure of their individual components. The pulsed electromagneticvalve operates at a pulse length of 2 ms; the length of the expansionchannel is from two mm to two cm and its diameter is from at least 100μm to at least one mm; the supersonic nozzle has a conical opening angle2θ between 2° and 20°, an input opening diameter larger than 100 nm anda conical section of a length from 2 to 10 mm. After pressing the targetliquid upon opening of the valve into the expansion channel where as aresult of its being heated a supersaturated water vapor is present, itwill expand during passage through the ultrasonic nozzle, cool, and formliquid droplets of the desired size and density, the parameters beingdetermined by the dimensions of the expansion channel, its temperatureand the prevailing pressure in it.

The method in accordance with the invention includes the followingmethod steps: Filling of a target liquid into a container, in which ahigh pressure is generated by means of a non-reactive gas, brief openingof the receptacle by a pulsed electromagnetic valve, pulsed introductionof the target liquid into an expansion channel, heating of the expansionchannel such that a supersaturated liquid vapor is generated, cooling ofthe vapor during passage to a supersonic valve connected to theexpansion channel, discharge of the droplets from the output opening ofthe nozzle into a vacuum.

In some embodiments of the inventive method a pulsed electromagnet valveis used operating in the ms range and, more particularly, at a pulseduration of 2 ms. At each switching of the valve the target liquid ispressed into the expansion channel and the corresponding vapor ispressed into the supersonic nozzle. An expansion channel of from 2 mm to2 cm in length and a diameter of at least 100 μm to at least one mm anda supersonic nozzle with a conical opening angle 2θ between 2° and 20°,an input opening diameter larger than 100 μm and a conically shapedsection between two and ten mm in length are used. During its passage tothe discharge opening of the nozzle the supersaturated gas is cooled inthe nozzle. This leads to the formation of liquid droplets. It isfurther to be mentioned that in addition to the mentioned parameters ofthe expansion channel the diameter of the nozzle also determines thediameter of the liquid droplets emerging from the nozzle opening into avacuum.

Compared to the prior art which constitutes the basis of the invention,the valve in accordance with the invention regulates the direct feedinginto an additionally provided expansion channel in which the targetliquid is heated. The thus present supersaturated gas is fed to thedischarge opening of the nozzle and cooled causing droplets to be formedin the nozzle. By contrast, in the prior art arrangement, the valveswitches the nozzle directly into its closed and open states whichsubstantially lessens the effect on the formation and extent of thedroplets and their collimation.

DESCRIPTION OF THE SEVERAL DRAWINGS

The novel features which are considered to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, in respect of its structure, construction andlay-out as well as manufacturing techniques, together with other objectsand advantages thereof, will be best understood from the followingdescription of preferred embodiments when read in connection with theappended drawings, in which:

FIG. 1 schematically depict the structure of an apparatus in accordancewith the invention;

FIG. 2 is a curve of the switching pulse of the valve and the associatedintensity of the liquid spray generated as a function of time;

FIG. 3 is a curve of the width of expansion of the liquid spray in airand in vacuum as a function of the distance from the discharge openingof the nozzle;

FIG. 4 is a curve of the density of the liquid spray as a function ofthe distance from the discharge opening of the nozzle; and

FIG. 5 is a curve of the relative intensity of scattered light measuredby CCD.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The apparatus in accordance with the invention for generating a droplettarget is provided with a pulsed electromagnetic valve 1. The valvecloses a receptacle 6, in which target liquid is maintained at apressure of 35 bar by gaseous nitrogen. The target liquid may be water,but in principle it may be any other liquid as well. The valve 1 opensand closes at a pulse duration of 2 ms and, in its open phase,discharges water droplets into an expansion channel 2 of 1 mm diameterand 15 mm length. By means of a heater 3 a temperature of 150° C. isgenerated in the expansion channel 2. The expansion channel 2 isseparated from the valve 1 by an insulator 5. The supersaturated watervapor present at the end of the expansion channel 2 is then fed througha supersonic nozzle 4. The nozzle 4 has an opening angle of 2θ=7°, aninput opening of 500 μm in diameter and a conical section of 8 mm lengthand generates sub-μ liquid droplets into the vacuum. At the dischargeopening of the supersonic nozzle 4, there is formed a droplet targetwhich can be generated continuously and which makes possible anoperation of unlimited duration.

FIG. 2 displays a curve of the switching pulse of the valve and theassociated intensity of the generated liquid spray as a function of timeat a distance of 1 mm from the discharge opening of the nozzle. In thismeasurement during which the radiation generated by a cw He—Ne-laser wasdirected to and scattered by the droplet target, and the intensity ofthe scattered radiation at a spacing of 1 mm from the nozzle opening wasdetermined, the pulse duration of the valve was 2 ms. It can be seenthat the major portion of the spray pulse occurs about 1 ms afteropening of the valve.

FIG. 3 shows a curve depicting the spread of the liquid spray as afunction of distance from the discharge opening of the nozzle in air andin vacuum. Compared to results known from the prior art, it can be seenthat the collimation resulting in accordance with the invention isimproved by about 30%.

The spread geometry of the generated cloud of droplet spray may bedefined as R=(0.32±0.02)×h+r, R being the radius of the spray/mistcloud, h being the distance from the supersonic nozzle and r being theradius of the discharge opening of the supersonic nozzle. A zerodistance corresponds to the discharge opening of the supersonic nozzle.

FIG. 4 discloses a curve which depicts the dependency of the density ofthe droplets within the spray as well as the dependency of the meanatomic density in the spray upon the distance from the discharge openingof the nozzle. The measured droplet density varies as regards dropletsof a 0.15 μm diameter from (1.6±0.5)·10¹¹ droplets per cubic centimeter(or a mean molecular density of 1.5·10¹⁸ cm⁻³) directly at the dischargeopening of the nozzle to (7.5±0.7)·10⁹ droplets/cm⁻³ (or mean moleculardensity of 8·10¹⁶ cm⁻³) at a distance of 20 mm from the dischargeopening. At this droplet size this constitutes a droplet density higherby up to three orders of magnitude than in currently described spraydroplet sources. This is important for the conversion of irradiatedlaser energy.

FIG. 5 depicts the measurement data of the scattered light intensity asa function of the viewing angle. The solid line represents thetheoretical distribution of the scattered light intensity of particlesof a diameter of 0.15 μm. The correspondence with the measurement dataindicates a closer distribution of the droplet sizes than in the priorart so that—unlike in the prior art—there is no need for a droplet sizefilter and that in this manner the effective droplet density isadvantageously increased.

1. An apparatus for generating a droplet target, comprising: at leastone receptacle for receiving a target liquid and adapted to have itsinterior maintained under high pressure; an electromagnetic valveswitching between open and closed states by pulses in the range of ms;means for feeding target liquid to the valve from the receptacle; asupersonic nozzle; an expansion channel for feeding target liquid fromthe valve to the nozzle; heating means associated with the expansionchannel for converting target liquid therein to supersaturated vapor bya predetermined temperature; and insulating means between theelectromagnetic valve and the heating means.
 2. The apparatus of claim1, wherein the pressure is maintained by a non-reactive gas.
 3. Theapparatus of claim 2, wherein the non-reactive gas is nitrogen.
 4. Theapparatus of claim 1, wherein the predetermined temperature is about150° C.
 5. The apparatus of claim 1, wherein the duration of the pulsesis 2 ms.
 6. The apparatus of claim 1, wherein the expansion channel isof a length from two mm to at least 20 mm and of a diameter of from atleast 100 μm to at least one mm.
 7. The apparatus of claim 6, whereinthe length is 15 mm and the diameter is 1 mm.
 8. The apparatus of claim1, wherein the supersonic nozzle is provided with a conical openingangle 2θ of between 2° and 20°, an input opening larger than 100 μm indiameter and a conically shaped section of a length of between 2 and 10mm.
 9. The apparatus of claim 8, wherein the opening angle is 7°, thediameter is 500 μm and the length of the conically shaped section is 8mm.
 10. A method of making a droplet target, comprising the steps of:filling a receptacle with a target liquid; maintaining a predeterminedpressure within the receptacle; briefly opening the receptacle by meansof a pulsed electromagnetic valve; feeding the target liquid through theelectromagnetic valve into an expansion channel; heating the expansionchannel to a temperature sufficient to convert the target liquid into asupersaturated vapor; feeding the supersaturated vapor to a supersonicnozzle; cooling the supersaturated vapor passing to the nozzle tocondense to droplets; and discharging the droplets from the nozzle. 11.The method of claim 10 wherein the pressure is maintained by gaseousnitrogen at 35 bar and the valve is pulsed at 2 ms.
 12. The method ofclaim 10, wherein the supersaturated vapor is fed to an expansionchannel of a length of from two mm to at least 20 mm and a diameter offrom at least 100 μm to at least one mm.
 13. The method of claim 10,wherein the supersaturated vapor is fed into and is cooled in asupersonic nozzle having a conical opening angle 2θ between 2° and 20°and a conically shaped section of a length of 2 to 10 mm.